1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same. The present invention is suitable for a video camera, digital camera, a TV camera, and a surveillance camera.
2. Description of the Related Art
A recent image pickup apparatus including an image sensor, such as a video camera and a digital camera, is required to have a compact size and a high performance. Hence, an image pickup optical system used for the image pickup apparatus is demanded for a high optical performance zoom lens that provides a wide angle of view, a high zooming ratio, and a compact overall system size. A negative lead type zoom lens in which a lens unit of a negative refractive power is arranged on the side closest to the object is known as a zoom lens that provides a wide angle of view and a compact overall system size.
It is easy for the negative lead type zoom lens to obtain a wide image pickup angle of view at the wide angle end, and to miniaturize the whole zoom lens because the lens unit of the negative refractive power on the side closest to the object has a compact configuration. Known negative lead type zoom lenses are a three-unit zoom lens and a four-unit zoom lens each of which includes, in order from the object side to the image side, a first lens unit of a negative refractive power, a second lens unit of a positive refractive power, and a third lens unit of a positive refractive power.
A three-unit zoom lens disclosed in each of U.S. Pat. No. 8,031,410, Japanese Patent Laid-Open No. 2010-060894, and U.S. Pat. No. 7,023,623 includes a first lens unit that includes a negative lens and a positive lens, a second lens unit that includes a positive lens, a cemented lens in which a positive lens and a negative lens are joined together, and a positive lens, and a third lens unit that includes a positive lens. U.S. Pat. No. 7,042,650 discloses a four-unit zoom lens as another negative lead type zoom lens which includes, in order from the object side to the image side, a first lens unit of a negative refractive power, a second lens unit of a positive refractive power, a third lens unit of a positive refractive power, and a fourth lens unit of a positive refractive power.
One method for realizing a zoom lens having a high resolving power is to correct an aberration in accordance with a sensitivity wavelength distribution (spectral sensitivity characteristic) of the image sensor. A signal detected by the image sensor is composed of red (R), green (G), and blue (B) signals.
A general image sensor has a sensitivity wavelength distribution illustrated in FIG. 12 for each of the RGB signals, and particularly has a high sensitivity in the wavelength corresponding to the G signal band. It is thus important for a zoom lens having a high resolving power to properly correct the chromatic aberration in a wavelength range from the F-line (486.13 nm) to d-line (587.56 nm) corresponding to the wavelength of the G signal band in which the image sensor has a high sensitivity. In particular, the lateral chromatic aberration among the chromatic aberration remarkably affects the resolving power of the whole screen, and thus it is particularly important to properly correct the lateral chromatic aberration in the entire zooming range.
In the negative lead type three-unit or four-unit zoom lens, it is important to properly determine a configuration of each lens unit so as to realize a compact overall system size and a wide angle of view, to properly correct the lateral chromatic aberration, and to obtain high optical performance. For example, unless the first lens unit has a proper lens configuration or a proper material, such as an Abbe number and a partial dispersion ratio, of each lens, it is difficult to realize a compact overall system size and a wide angle of view, and to obtain high optical performance. This is not limited to the negative lead type three-unit or four-unit zoom lens but is true of the negative lead type zoom lens having more lens units.